When combustion of hydrocarbon fuel in internal combustion engines and furnaces is ideal and complete, the combustion products and exhaust include carbon dioxide and water. However, due to incomplete combustion unburned hydrocarbons, partially burned hydrocarbons, and thermal crack products, as well as nitrogen and sulfur containing byproducts may be present in the exhaust. Some components of exhaust, such as carbon monoxide, hydrocarbons, and nitrogen monoxide (which converts to nitrogen dioxide), are believed to adversely affect the environment as well as humans. Thus, many components of exhaust gas, including non-methane hydrocarbons, are now regulated.
One conventional method of determining non-methane hydrocarbon concentration in engine exhaust or dilute engine exhaust includes the separate measurement of total hydrocarbons and methane in the exhaust. The concentration of non-methane hydrocarbons is then determined from the difference between the total hydrocarbon concentration and methane concentration. However, this method is indirect and a variety of factors influence the measurement of these values. Such factors include engine technology, methods used to measure the total hydrocarbon concentration and methane concentration, and the ratio of total hydrocarbon concentration to methane concentration. These factors can lead to errors and negative non-methane hydrocarbon emission values. This is particularly the case when methane concentration is close in value to total hydrocarbon concentration.
Direct measurements using gas chromatography (GC) have also been used to determine non-methane hydrocarbon concentration in exhaust gas. During GC, the various constituents (i.e., methane, non-methane hydrocarbons, etc.) are separated out of the exhaust gas. However, this process is relatively expensive and time consuming. Furthermore, the method is dependent on a number of factors, such as the flow rate of the analyte through the column and the size and composition of the stationary phase, etc.
Thus room remains for improvement in the measurement of non-methane hydrocarbon concentration present in exhaust gas.